If you have a thermometer with 1 degree centigrade divisions, it is reasonable to say that the thermometer has a resolution of 1c, or +/- 0.5c If the accuracy stated by the manufacturer is 0.5c, then the combined error margin is +/- 1.0c. In this case all temperatures recorded with this thermometer should be rounded up to the nearest 1c. It is impossible to measure 1.5c or 1.75c etc, even if it appears you can do so when you are looking at the thermometer. Even then, you should still remember that your observed recording is +/- 1c.
(need to expand on this at some point)